Very Dim Q - re custody and divorce

[verydim]

I have been divorced for 5 years now

but I cant find anything in writing about custody

would it be worded as custody in normal divorce proceedings or is a custody case seen separate?

I remember signing something saying that all medical/educational/care decisions are my choice

I have 2 big files of paperwork about the divorce but the most recent and important looking state that X agrees DS resides with me and access will be agreed between ourselves

So technically I have sole custody but legally?

As I said, very dim as I know it was the only important thing at the time but now I cant seem to see it in writing

Unless you actually went to court with regards to the issue of residence and contact (formerly called custody and access) then you wouldn't actually have any papers regarding this. This is because since the Children Act 1989 the Court has the 'no order' principal, in that it will only make an order if it is absolutely necessary and in the best interests of the child/children. Therefore, if there was no dispute with your ex that your DS should reside with you, then there would be no orders relating to residence or contact. Previously, one parent had to have a 'custody order' and the other an 'access order' which is not the case now.
It sounds as if the papers you are referring to that said DS would reside with you is the 'statement of arrangements for children' form which would have been lodged at the court together with the divorce petition at the time of the divorce. This form simply sets out what the current, and future arrangements are for any children of the marriage and would normally set out information such as the type of property the child lives in and the current arrangements for residence and contact with the non-resident parent.
Is there now an issue with regards to DS's residence? If ex is now suggesting that DS should live with him (and you are not happy with this) then he would have to make an application for a Residence Order through the County Court. Bearing in mind however that DS has been with you for over 5 years it is likely that if such an application was made by your ex the Court would usually say that the status quo should be maintained (ie DS should remain living with you) Hope this helps.